Designs of permanent magnet arrays aiming at achieving a strong and uniform magnetic field have been previously reported in the patent literature. For example, U.S. Pat. No. 7,423,431 describes a permanent magnet assembly for an imaging apparatus having a permanent magnet body having a first surface and a stepped second surface which is adapted to face an imaging volume of the imaging apparatus, wherein the stepped second surface contains at least four steps.
As another example, U.S. Pat. No. 6,411,187 describes adjustable hybrid magnetic apparatus for use in medical and other applications includes an electromagnet flux generator for generating a first magnetic field in an imaging volume, and permanent magnet assemblies for generating a second magnetic field superimposed on the first magnetic field for providing a substantially homogenous magnetic field having improved magnitude within the imaging volume. The permanent magnet assemblies may include a plurality of annular or disc like concentric magnets spaced-apart along their axis of symmetry. The hybrid magnetic apparatus may include a high magnetic permeability yoke for increasing the intensity of the magnetic field in the imaging volume of the hybrid magnetic apparatus.
U.S. Pat. No. 10,018,694 describes a magnet assembly for a magnetic resonance imaging (MRI) instrument, the magnet assembly comprising a plurality of magnet segments that are arranged in two or more rings such that the magnet segments are evenly spaced apart from adjacent magnet segments in the same ring, and spaced apart from magnet segments in adjacent rings. According to an embodiment, a plurality of magnet segments is arranged in two or more rings with the magnetization directions of at least some of the magnet segments being unaligned with a plane defined by their respective ring, to provide greater control over the resulting magnetic field profile.
U.S. Pat. No. 5,900,793 describes assemblies consisting of a plurality of annular concentric magnets spaced-apart along their axis of symmetry, and a method for constructing such assemblies using equiangular segments that are permanently magnetized.